The process of synthesizing nanostructures such as Carbon Nanotubes (CNTs) by chemical vapor deposition (CVD), or more generally by a mechanism such as the Vapor-Liquid-Solid (VLS) mechanism is widely known in the field of materials science, involves growth of a rod or tube-like solid nanostructure from a catalyst or seed particle. The particle assists in the decomposition of reactive precursors, and under appropriate conditions the nanostructure grows from the particle. Rod or tube-like nanostructures have been grown by this method to have exceptional aspect ratios. While these nanostructures and the seed particles are commonly only nanometers (e.g., 1-100 nm) in diameter, reaction zones in traditional growth systems for nanomaterials, such as tube furnaces, are centimeters to meters in size. In these systems, the growth reactions are mediated in part by the flow rates, pressure, and temperature established at the macro-scales of the growth chambers, rather than the micro- and nano-scales of the growth processes. In addition, because the forces experienced by the nanostructures during growth are dominated by phenomena such as thermal vibration, free convection, or surface interactions, growth systems are not capable of efficiently directing the growth and assembly of individual nanostructures or groups of nanostructures into ordered macroscopic configurations, such as fibers with exceptional mechanical properties. Often, the only localized control of growth conditions is achieved by the location of the catalyst particles, which determine where growth initiates and where the growth reaction occurs. While recent developments have shown that these systems can produce bulk quantities of nanostructures, these systems fail to produce assemblies of nanostructures having properties approaching those of individual nanostructures. Furthermore, proposed methods of assembly by post-processing of bulk quantities of nanostructures, such as spinning solutions of carbon nanotubes into or directly assembling CNTs end-to-end into fibers or sheets by dry methods have only seen moderate success because the individual nanostructures are not sufficiently long, aligned, and connected for sufficient transmission of mechanical loads.
Since before the concentric-graphene-layer structure of carbon nanotubes (CNTs) was confirmed, the applications of CNT-based materials have been desired, yet production of materials containing a high weight fraction of CNTs, such as CNT fibers, has not been realized at large scales or at reasonable cost. CNTs have many times the normalized stiffness and strength of steel, are very flexible, and have outstanding electrical and thermal properties. To be commercially successful, CNT-based materials must replicate the properties of individual CNTs. Today, systems for bulk CNT growth often produce tangled nanotubes, bundles, and agglomerates in large reaction chambers.
Chemical vapor deposition (CVD) is the most successful method of producing CNTs, whereby CNTs precipitate from nanometer-scale metal particles in a hydrocarbon gas (e.g., methane, ethylene, acetylene, alcohol vapor) environment, because the metals (e.g., Fe, Ni, Co) catalyze decomposition of hydrocarbons at temperatures below those at which the hydrocarbons self-pyrolyze. CVD growth of CNTs typically occurs at up to 1300 degrees Celsius, when the catalytic reaction is energetically favorable. The nanotube diameter is roughly equal to the diameter of the catalyst particle.
The sciences of preparing and characterizing catalysts for CNT growth, and understanding the reaction mechanisms between the catalysts, support materials, and different chemicals used to grow CNTs, are wide areas within the CNT and nanomaterials research fields. The most common embodiment of CVD growth of CNTs involves placing a substrate such as a piece of a silicon wafer, coated with a nanostructured metal film which contains catalyst particles, inside a reaction chamber such as a tube furnace.
There are a variety of typical furnace configurations for CVD growth of CNTs, such as horizontal and vertical tube furnaces, and vacuum chambers. In a “floating” or “fluidized” catalyst system, catalyst particles are typically introduced in the gas phase, and CNTs grow from these suspended catalyst sites. Floating catalyst mixtures, for example using ferrocene dissolved in toluene or acetylene also produce dense films of well-aligned nanotubes on quartz (oxide) surfaces (e.g., the inside of a quartz tube) because the catalyst particles adhere to oxide. Delivery of the catalyst in the gas phase, rather than pre-patterning of metal the substrate, can significantly reduce the preparation time required for growing CNT films. Furthermore, because many more catalyst sites can be active in a gas volume than on a small substrate coated with a catalyst film, and because of continuous operation, floating catalyst methods are more attractive for synthesizing bulk quantities of CNTs. However, with the exception of carefully-designed thermal and plasma-enhanced methods for producing vertically-aligned CNT arrays and the direct dry spinning methods, these CVD techniques produce a majority of tangled CNTs which must be sorted and separated before being useful for applications in composite materials.
While CNT growth from floating catalytic sites is readily more scalable for bulk synthesis, growth from substrates offers greater control of the arrangement, density, and length of CNTs. Typically, growth of aligned CNTs on a substrate is achieved using an unsaturated hydrocarbon gas such as acetylene or ethylene and a predeposited catalyst film or by evaporating a solution of an organometallic catalyst precursor and a liquid hydrocarbon. Millimeter-scale architectures of aligned multi-wall CNTs (MWNTs) have been routinely achieved using floating catalyst precursors; however, these structures typically have lesser crystallinity and higher impurity content (e.g., metal catalyst periodically along the tube axis) than structures grown from pre-deposited catalyst films. Further, by adding an optimal amount of ferrocene to acetylene feedstock during growth, increased the terminal length of VA-MWNTs, grown from a Al/Fe/Mo catalyst film, from less than 0.5 mm to greater than 3 mm. This is attributed to ferrocene increasing the effectiveness of ethylene by promoting its dehydrogenation before reaching the catalyst site. Further study has shown that homogeneous gas phase reactions critically affect the CNT growth process in many systems, and that a minimum incidence rate of carbon-containing molecules is necessary for vertically aligned growth. Overall, the balance between the decomposition and diffusion rates of active carbon at the catalyst is a major determinant of the CNT growth rate, and of the duration of growth before the reaction terminates. Recently, atmospheric pressure synthesis of vertically aligned single-wall CNTs (SWNTs) was reported, where introduction of a controlled concentration of water vapor gives rapid growth of high-purity SWNT films to a thickness of up to 2.5 mm, possibly due to selective removal of catalyst-bound amorphous carbon by water. Since, plasma-enhanced CVD growth of VA-SWNT and VA-DWNT films has been achieved, without using an oxygen-containing additive.
Growth of continuous CNT strands having macroscopic lengths is perhaps the only way to replicate the fantastic mechanical properties of individual CNTs in large-scale materials. Fundamental obstacles must be overcome before bulk production processes give exceptionally stiff and strong CNT materials. These issues include interconnection, alignment, and dense packing of CNTs for effective load transfer, and incorporation of high CNT loadings in matrix materials. Concomitantly, rapid growth rates have been achieved from catalyst particles situated on substrates, where self-alignment of CNTs into ordered structures can be achieved by suitable choice of the CVD conditions. To our knowledge, the highest reported CVD growth rate of a SWNT is 0.61 mm/min., while growth of carbon nanofibers has been observed at 10's of mm/min., and CNT growth by arc discharge occurs at up to 100 m/s for very short times. Only a moderate increase in the growth rate and active duration of fixed substrate CVD growth processes is necessary to produce centimeter or meter long continuous CNT strands.
Studies involving rapid heating and thermal cycling have shown that temperature magnitude and its temporal variation can influence the progress of chemical reactions. For example, formation of single-wall carbon nanotubes occurred only under rapid heating of a supported catalyst powder. Although the effects of unsteady conditions have been widely studied for other parameters such as reactant composition, rapid thermal control cannot be studied using traditional reactors due to their large thermal mass. Many high-temperature gas-phase reactions such as chemical vapor deposition (CVD) processes utilize tube furnaces, where the reaction sample (e.g., a substrate coated with a catalyst) is sealed inside a ceramic (e.g., quartz) tube, and the reactant atmosphere flows through the tube. A laboratory-scale tube furnace which is externally controlled using resistively-heated coils is typically limited to a heating rate of $\sim$10\,\dgc/min. Effects of rapid heating using tube furnaces have thus been studied by moving the sample or tube with respect to the heater coils or by injecting floating catalyst particles into the heated zone. Alternatively, electrically conductive substrates and/or particles can be directly heated inductively˜or by using microwave radiation. However, continuous temperature control of the reaction surface is not routinely achieved by these methods. Further, in traditional CVD setups, the substrate temperature is not directly measured, and is rather assumed to be equal to the temperature measured by a thermocouple outside the tube.
Traditional CNT growth chambers are customarily inches to meters in size, dwarfing the size of their products by several orders of magnitude. Alternatively, microreactors confine reactions to micron-millimeter scales, where very precise kinetic and thermal control can be achieved. For example, a device with 256 parallel channels etched in silicon was used to establish a uniform pressure drop across a packed bed of catalyst beads, synthesized CdSe nanocrystals and nanorods in etched glass microchannel reactors, and demonstrated that the size of the nanostructures could be controlled by changing the temperature of the device, which was heated by a thin-film heater attached to the bottom of the reactor chip. Furthermore, micro-scale fluid flows can be used to align and subsequently pattern nanowires on a substrate, flowing a suspension of pre-grown semiconducting nanowires through microchannels. The degree of alignment of the nanowires is related to the flow rate of the liquid through the channels, and after alignment the channel template is removed and the nanowires are anchored to the surface. Thin-film elements in microreactors, such as platinum-titanium traces, have been used as local heaters and temperature sensors, eliminating the need for expensive and bulky furnace systems. The only CNT example here is localized CVD growth in a room-temperature hydrocarbon ambient by resistive heating of a catalyst-coated polysilicon microbridge.
Resistive heating has been used to reach the growth temperature, by passing current through an electrically conductive growth substrate. For example: a piece of carbon paper; a metallic wire, a silicon substrate; or a microfabricated silicon bridge. On all these examples including the silicon nanobridge, the temperature is constant during growth.
In situ monitoring of CNT growth process has been used to gain insight into the time evolution of CNT films and structures and growth sites, in relation to the reaction conditions; however this monitoring has not been used to directly control the growth process in a feedback scheme. Examples of in-situ monitoring include optical measurement of film thickness and optical properties, Raman spectroscopy, and characterization of the catalyst by photoelectron spectroscopy and X-ray diffraction.